1. Field of the Invention
The present invention relates to an exhaust emission control device and method for an internal combustion engine and an engine control unit, and more particularly to an exhaust emission control device including a NOx catalyst that is disposed in the exhaust system of the engine, for trapping NOx in exhaust gases emitted from the engine, an exhaust emission control method, and an engine control unit.
2. Description of the Related Art
Conventionally, there has been proposed an exhaust emission control device of this kind, such as an exhaust emission control device for a diesel engine (hereinafter simply referred to as “the engine”), e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2002-371889. In this exhaust emission control device, to restore the NOx trapping performance of a NOx catalyst for regeneration of the NOx catalyst, a NOx-reducing control process in which NOx trapped by the NOx catalyst is reduced and the reduced NOx is released is carried out as follows: A target equivalent ratio of the air-fuel ratio is calculated such that it becomes equal to a rich value not smaller than 1, and a target intake air amount is calculated based on the calculated target equivalent ratio and the load on the engine. The intake air amount is controlled such that it becomes equal to the calculated target intake air amount. Further, the fuel injection amount of the engine is calculated according to the target equivalent ratio and a detected intake air amount. With the above-described NOx-reducing control process, the reducing condition of exhaust gases flowing into the NOx catalyst is controlled to a predetermined reducing condition, whereby NOx trapped by the NOx catalyst is reduced and the reduced NOX is released so as to regenerate the NOx catalyst.
Further, in a case where the target intake air amount is smaller than a minimum controllable intake air amount, and hence it is impossible to control the intake air amount to the target intake air amount and sufficiently control the exhaust gases to the reducing condition, the deficiency is compensated for by post injection, that is, injection of fuel during the expansion stroke or the exhaust stroke of the engine. Further, during a transient operation of the engine in which the difference between the target intake air amount and the intake air amount is large, the NOx-reducing control process is not performed, and the fuel injection amount is calculated not according to the intake air amount but according to the load on the engine. This prevents torque variation from occurring due to calculation of too large or too small a fuel injection amount, during the transient operation of the engine as described above.
Fuel injected during the expansion stroke or the exhaust stroke by the post injection is not burned in the engine but is burned in the exhaust system including the NOx catalyst, whereby the temperature of the NOx catalyst is increased. Therefore, as the amount of post-injected fuel (hereinafter referred to as “the post injection amount”) is increased, the temperature of the NOx catalyst is increased. On the other hand, in the conventional exhaust emission control device, fuel is post-injected in the fuel injection amount calculated according to the intake air amount during execution of the NOx-reducing control process, as described above. As a result, e.g. when the temperature of the NOx catalyst is very high, if the load on the engine increases to increase the intake air amount, there is a fear that the post injection amount is accordingly increased to make the temperature of the NOx catalyst so high as will cause the NOx catalyst to be damaged by melting.
Further, the NOx catalyst of this kind has the following characteristics: The NOx catalyst traps not only NOx but also SOx contained in exhaust gases, and as the amount of the trapped SOx becomes larger, the NOx trapping performance thereof is lowered. Further, when the temperature thereof is within a predetermined relatively high temperature range, and at the same time the exhaust gases are in a predetermined reducing condition, the NOx catalyst reduces the trapped SOx and releases the reduced SOx. In view of these characteristics, there has been proposed a SOx-reducing control process in which the trapped SOx is caused to be reduced and the reduced SOx is released e.g. by controlling the temperature of the NOx catalyst so as to thereby regenerate the NOx catalyst.
If the SOx-reducing control process is carried out in the same manner as the above-described NOx-reducing control process by the conventional exhaust emission control device, fuel is post-injected in the fuel injection amount calculated according to the intake air amount. Therefore, during execution of the SOx-reducing control process, as the intake air amount is reduced, the post injection amount is reduced, so that the temperature of the NOx catalyst sometimes becomes lower than the above-described predetermined temperature range. In this case, it is impossible to properly reduce SOx and cause the reduced SOx to be released, which makes it impossible to properly regenerate the NOx catalyst.